Faculty of Medicine Introduction to Community Medicine Course

(31505201)

Introduction to Epidemiology

By

Hatim Jaber MD MPH JBCM PhD

20 - 11- 2017

1

Introduction to course 31505201 Course Content- Post midterm

bull Week 8 Unit 6 Demography and Data bull Week 9 Midterm assessment (Exams)15-11-2017

bull Week 10 Unit 8 Introduction to Epidemiology

bull Week 11 Unit 9 Causation Prevention and Control of Diseases bull Week 12 Unit 10 Health Education and Communication bull Week 13 Unit 11 Public Health Surveillance and Screening bull Week 14 Unit 12 Health Administration and healthcare management bull Week 15 Unit 13 Revision and Health Research

bull Week 16 Final assessment (Exams)

Today is World Toilet Day

Access to sanitation is an important right that decreases the spread of diseases such as diarrhea

Sustainable Development Goals launched in 2015 include a target to ensure that everyone everywhere has access to toilets by 2030

bull Although much progress has been made almost 750 million people in the WHO South-East Asia Region still do not have access to improved sanitation

bull Introduction to Epidemiology Definition History of Epidemiology PurposeUse of Epidemiology Concepts in the infectious diseases

bull Measurements of Morbidity and Mortality

Presentation outline

Time

Introduction to Epidemiology 0800 to 0815

Public Health amp Epidemiology 0815 to 0830

Key Words 0830 to 0845

Main components 0845 to 0900

Concepts in the infectious diseases 0900 to 0915

6

Epidemiology

bull Epi = upon

bull Demos = population

bull Logos = study of

Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Epidemiological Principles

bull bullDiseases (or other health events) donrsquot occur at random

bull bullDiseases (or other health events) have causal and preventive factors which can be identified

bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than

individual persons tissues or organs

bull laquo The art of epidemiological thinking is to

draw conclusions from imperfect data raquo

George W Comstock

Epidemiology and Preventive Medicine

bull Epidemiology is the central science of public

health and preventive medicine is a clinical approach to public health practice

bull Epidemiology can provide the preventive medicine practitioner with the following

bull - Information on the state of the health of the population

bull - Methods for identifying possible determinants of health and disease within individuals

bull - Appropriate population groups for interventions bull - Understanding the origins of public health

recommendations

Why have chronic diseases increased in importance

bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases

bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)

bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-

communicable) bull ndashChange in lifestyles (access amp economy)

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Introduction to course 31505201 Course Content- Post midterm

bull Week 8 Unit 6 Demography and Data bull Week 9 Midterm assessment (Exams)15-11-2017

bull Week 10 Unit 8 Introduction to Epidemiology

bull Week 11 Unit 9 Causation Prevention and Control of Diseases bull Week 12 Unit 10 Health Education and Communication bull Week 13 Unit 11 Public Health Surveillance and Screening bull Week 14 Unit 12 Health Administration and healthcare management bull Week 15 Unit 13 Revision and Health Research

bull Week 16 Final assessment (Exams)

Today is World Toilet Day

Access to sanitation is an important right that decreases the spread of diseases such as diarrhea

Sustainable Development Goals launched in 2015 include a target to ensure that everyone everywhere has access to toilets by 2030

bull Although much progress has been made almost 750 million people in the WHO South-East Asia Region still do not have access to improved sanitation

bull Introduction to Epidemiology Definition History of Epidemiology PurposeUse of Epidemiology Concepts in the infectious diseases

bull Measurements of Morbidity and Mortality

Presentation outline

Time

Introduction to Epidemiology 0800 to 0815

Public Health amp Epidemiology 0815 to 0830

Key Words 0830 to 0845

Main components 0845 to 0900

Concepts in the infectious diseases 0900 to 0915

6

Epidemiology

bull Epi = upon

bull Demos = population

bull Logos = study of

Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Epidemiological Principles

bull bullDiseases (or other health events) donrsquot occur at random

bull bullDiseases (or other health events) have causal and preventive factors which can be identified

bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than

individual persons tissues or organs

bull laquo The art of epidemiological thinking is to

draw conclusions from imperfect data raquo

George W Comstock

Epidemiology and Preventive Medicine

bull Epidemiology is the central science of public

health and preventive medicine is a clinical approach to public health practice

bull Epidemiology can provide the preventive medicine practitioner with the following

bull - Information on the state of the health of the population

bull - Methods for identifying possible determinants of health and disease within individuals

bull - Appropriate population groups for interventions bull - Understanding the origins of public health

recommendations

Why have chronic diseases increased in importance

bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases

bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)

bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-

communicable) bull ndashChange in lifestyles (access amp economy)

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Today is World Toilet Day

Access to sanitation is an important right that decreases the spread of diseases such as diarrhea

Sustainable Development Goals launched in 2015 include a target to ensure that everyone everywhere has access to toilets by 2030

bull Although much progress has been made almost 750 million people in the WHO South-East Asia Region still do not have access to improved sanitation

bull Introduction to Epidemiology Definition History of Epidemiology PurposeUse of Epidemiology Concepts in the infectious diseases

bull Measurements of Morbidity and Mortality

Presentation outline

Time

Introduction to Epidemiology 0800 to 0815

Public Health amp Epidemiology 0815 to 0830

Key Words 0830 to 0845

Main components 0845 to 0900

Concepts in the infectious diseases 0900 to 0915

6

Epidemiology

bull Epi = upon

bull Demos = population

bull Logos = study of

Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Epidemiological Principles

bull bullDiseases (or other health events) donrsquot occur at random

bull bullDiseases (or other health events) have causal and preventive factors which can be identified

bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than

individual persons tissues or organs

bull laquo The art of epidemiological thinking is to

draw conclusions from imperfect data raquo

George W Comstock

Epidemiology and Preventive Medicine

bull Epidemiology is the central science of public

health and preventive medicine is a clinical approach to public health practice

bull Epidemiology can provide the preventive medicine practitioner with the following

bull - Information on the state of the health of the population

bull - Methods for identifying possible determinants of health and disease within individuals

bull - Appropriate population groups for interventions bull - Understanding the origins of public health

recommendations

Why have chronic diseases increased in importance

bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases

bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)

bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-

communicable) bull ndashChange in lifestyles (access amp economy)

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull Introduction to Epidemiology Definition History of Epidemiology PurposeUse of Epidemiology Concepts in the infectious diseases

bull Measurements of Morbidity and Mortality

Presentation outline

Time

Introduction to Epidemiology 0800 to 0815

Public Health amp Epidemiology 0815 to 0830

Key Words 0830 to 0845

Main components 0845 to 0900

Concepts in the infectious diseases 0900 to 0915

6

Epidemiology

bull Epi = upon

bull Demos = population

bull Logos = study of

Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Epidemiological Principles

bull bullDiseases (or other health events) donrsquot occur at random

bull bullDiseases (or other health events) have causal and preventive factors which can be identified

bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than

individual persons tissues or organs

bull laquo The art of epidemiological thinking is to

draw conclusions from imperfect data raquo

George W Comstock

Epidemiology and Preventive Medicine

bull Epidemiology is the central science of public

health and preventive medicine is a clinical approach to public health practice

bull Epidemiology can provide the preventive medicine practitioner with the following

bull - Information on the state of the health of the population

bull - Methods for identifying possible determinants of health and disease within individuals

bull - Appropriate population groups for interventions bull - Understanding the origins of public health

recommendations

Why have chronic diseases increased in importance

bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases

bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)

bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-

communicable) bull ndashChange in lifestyles (access amp economy)

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Presentation outline

Time

Introduction to Epidemiology 0800 to 0815

Public Health amp Epidemiology 0815 to 0830

Key Words 0830 to 0845

Main components 0845 to 0900

Concepts in the infectious diseases 0900 to 0915

6

Epidemiology

bull Epi = upon

bull Demos = population

bull Logos = study of

Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Epidemiological Principles

bull bullDiseases (or other health events) donrsquot occur at random

bull bullDiseases (or other health events) have causal and preventive factors which can be identified

bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than

individual persons tissues or organs

bull laquo The art of epidemiological thinking is to

draw conclusions from imperfect data raquo

George W Comstock

Epidemiology and Preventive Medicine

bull Epidemiology is the central science of public

health and preventive medicine is a clinical approach to public health practice

bull Epidemiology can provide the preventive medicine practitioner with the following

bull - Information on the state of the health of the population

bull - Methods for identifying possible determinants of health and disease within individuals

bull - Appropriate population groups for interventions bull - Understanding the origins of public health

recommendations

Why have chronic diseases increased in importance

bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases

bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)

bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-

communicable) bull ndashChange in lifestyles (access amp economy)

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Epidemiology

bull Epi = upon

bull Demos = population

bull Logos = study of

Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Epidemiological Principles

bull bullDiseases (or other health events) donrsquot occur at random

bull bullDiseases (or other health events) have causal and preventive factors which can be identified

bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than

individual persons tissues or organs

bull laquo The art of epidemiological thinking is to

draw conclusions from imperfect data raquo

George W Comstock

Epidemiology and Preventive Medicine

bull Epidemiology is the central science of public

health and preventive medicine is a clinical approach to public health practice

bull Epidemiology can provide the preventive medicine practitioner with the following

bull - Information on the state of the health of the population

bull - Methods for identifying possible determinants of health and disease within individuals

bull - Appropriate population groups for interventions bull - Understanding the origins of public health

recommendations

Why have chronic diseases increased in importance

bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases

bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)

bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-

communicable) bull ndashChange in lifestyles (access amp economy)

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Epidemiological Principles

bull bullDiseases (or other health events) donrsquot occur at random

bull bullDiseases (or other health events) have causal and preventive factors which can be identified

bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than

individual persons tissues or organs

bull laquo The art of epidemiological thinking is to

draw conclusions from imperfect data raquo

George W Comstock

Epidemiology and Preventive Medicine

bull Epidemiology is the central science of public

health and preventive medicine is a clinical approach to public health practice

bull Epidemiology can provide the preventive medicine practitioner with the following

bull - Information on the state of the health of the population

bull - Methods for identifying possible determinants of health and disease within individuals

bull - Appropriate population groups for interventions bull - Understanding the origins of public health

recommendations

Why have chronic diseases increased in importance

bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases

bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)

bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-

communicable) bull ndashChange in lifestyles (access amp economy)

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull laquo The art of epidemiological thinking is to

draw conclusions from imperfect data raquo

George W Comstock

Epidemiology and Preventive Medicine

bull Epidemiology is the central science of public

health and preventive medicine is a clinical approach to public health practice

bull Epidemiology can provide the preventive medicine practitioner with the following

bull - Information on the state of the health of the population

bull - Methods for identifying possible determinants of health and disease within individuals

bull - Appropriate population groups for interventions bull - Understanding the origins of public health

recommendations

Why have chronic diseases increased in importance

bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases

bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)

bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-

communicable) bull ndashChange in lifestyles (access amp economy)

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Epidemiology and Preventive Medicine

bull Epidemiology is the central science of public

health and preventive medicine is a clinical approach to public health practice

bull Epidemiology can provide the preventive medicine practitioner with the following

bull - Information on the state of the health of the population

bull - Methods for identifying possible determinants of health and disease within individuals

bull - Appropriate population groups for interventions bull - Understanding the origins of public health

recommendations

Why have chronic diseases increased in importance

bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases

bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)

bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-

communicable) bull ndashChange in lifestyles (access amp economy)

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Why have chronic diseases increased in importance

bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases

bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)

bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-

communicable) bull ndashChange in lifestyles (access amp economy)

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Defining characteristics of chronic diseases

bull bullProlonged course of illness

bull bullMultiple risk factors

bull bullLong latency period

bull bullNon-contagious origin (non-communicable)

bull bullUncertain etiology (causation)

bull bullFunctional impairment or disability

bull bullIncurability

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Epidemiology

Key Words of Definition

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Last 1988

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Distribution Time place person

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Determinants Cause risk factors

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Population = Public health

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Key Words

bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems

bull Application = Information for action

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Major components of the definition

bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals

bull For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die

bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a

quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates

bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc

bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Major components of the definition

bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease

bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Natural history of disease

bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention

bull There are four stages in the natural history of a disease These are

bull 1 Stage of susceptibility

bull 2 Stage of pre-symptomatic (sub-clinical) disease

bull 3 Stage of clinical disease

bull 4 Stage of recovery disability or death

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been

laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles

bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes

(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the

disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may

sometimes end in recovery without development of any signs or symptoms

Four stages in the natural history

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

3 The Clinical stage

bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death

bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost

everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop

paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost

always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and

eventually results in death if the patient is not properly treated

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

4 Stage of recovery disability or death

bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death

bull Disability is limitation of a persons activities including his role as a parent wage earner etc

Examples

bull bull Trachoma may cause blindness

bull bull Meningitis may result in blindness or deafness Meningitis may also result in death

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

1 Describe the health status of population

2 Explain the etiology of disease

3 Predict the frequency and distribution of disease

4 Control diseases in populations

Aims of Epidemiology

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Uses of Epidemiology

bull Determine the magnitude and trends

bull bullIdentify the etiology or cause of disease

bull bullDetermine the mode of transmission

bull bullIdentify risk factors or susceptibility

bull bullDetermine the role of the environment

bull bullEvaluate the impact of the control measures

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and

describe health problems in a community (for example the prevalence of anemia or the nutrition status of children)

bull To monitor continuously over a period of time the change of health in a

community (for example the effect of a vaccination program health education nutritional supplementation)

bull To practice surveillance for a specific disease in order to be able to act

quickly and so cut short any outbreak (example cholera)

bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak

bull To plan effective health services Effective services interventions and

remedies all depend on accurate community data

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Epidemiologist Core Functions

bull Public health surveillance

bull bullOutbreak investigations

bull bullData analysis

bull bullEvaluation of disease control programs

bull bullCommunication

bull bullManagement and teamwork

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Epidemiologic triad

Host

Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents

bullPhysical environment bullBiological environment bullSocial environment

bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Host Agent Environment

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Terminology and Definitions

bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence

of infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic

bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and

enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Terminology and Definitions (cont)

bull Virulence

bull Reproductive rate of infection

bull Host

bull Vector (source)

bull Reservoir

bull Incubation period

bull Infectivity period

bull Serial interval

bull Latent period

bull Transmission Probability ratio

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Infectious Diseases Epidemiology

Definition

bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Components of the infectious process

bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission

bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Components of the infectious process

I The Agents bull The agents in the infectious process range from

viral particles to complex multi-cellular organisms

II Reservoirs bull A reservoir is an organism or habitat in which an

infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Types of reservoirs

Reservoir

Human reservoir

Animal reservoir

Non-living reservoir

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Reservoir

bull Habitat in which the disease normally lives and multiplies

bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral

respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc

bull All infected humans whether showing signs and symptoms of the disease

or not are potential sources of infection to others

bull A person who does not have apparent clinical disease but is a potential

source of infection to other people is called a Carrier An example of

carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others

bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Components of the infectious process

III Portal of Exit

bull Portal of exit is the way the infectious agent leaves the reservoir

bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues

bull For example feces is the portal of exit for the eggs of hook worm

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

IV Mode of Transmission

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

IV Mode of Transmission

Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect

1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin

mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies

bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold

bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Indirect transmission bull 2 Indirect transmission The following are the different types of indirect

transmission

bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc

bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical

-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria

-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Indirect transmission

bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis

bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma

-Injury site is portal of entry for tetanus

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

VI Susceptible human host

bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level

bull Host resistance at the community (population) level is called herd immunity

bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible

bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull Disease Incidence Percentage of population that contracts a disease in a given time period

bull Disease Prevalence Percentage of population that has the disease during a given time period

Basic Epidemiological Terms

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Levels of disease

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull Sporadic Disease occurs only occasionally (ie Polio in US)

bull Endemic Disease constantly present in population (ie common cold or ear infection)

bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)

bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)

Basic Epidemiological Terms

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Epidemic

bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo

bull (epi= upon demos= people)

bull Epidemics can occur upon endemic states too

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Endemic

bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population

bull (En = in demos = people)

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Hyperendemic and holoendemic

bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally

bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Pandemic and Exotic

bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics

bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Sporadic

bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently

bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip

bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Attack rates and primarysecondary cases

bull Attack rate proportion of non-immune exposed individuals who become clinically ill

bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Zoonosis epizootic and enzootic

bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip

bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever

bull An Enzotic is an endemic occurring in animals eg bovine TB

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Nosocomial infections

bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility

bull It has to be a new disorder unrelated to the patientrsquos primary condition

bull Examples include infection of surgical wounds hepatitis B and urinary tract infections

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Opportunistic infection

bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease

bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus

bull M tuberculosishellip

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull The Host is the organism that is susceptible to the effect of the agent

bull The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness

Basic Epidemiological Terms

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Cases

bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Virulence and Case Fatality Rate

bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate

bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

The Nature of Infectious Disease

bull Virulence Factors of Infectious Agents ndash Pathogenicity

bull Ability of a microorganism to cause disease

ndash Virulence bull Degree of pathogenicity

bull Virulence factors contribute to virulence ndash Adhesion factors

ndash Biofilms

ndash Extracellular enzymes

ndash Toxins

ndash Antiphagocytic factors

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Serial interval and Infectious period

bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)

bull Infectious (communicable) period length

of time a person can transmit disease (sheds the infectious agent)

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Incubation and Latent periods

bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question

bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized

Basic Epidemiological Terms

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source

Basic Epidemiological Terms

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play

Basic Epidemiological Terms

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Carriers

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Human reservoir

Human reservoir

cases carriers

According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases

bullPrimary case bullIndex case bullSecondary cases

Type bullIncubatory bullConvalescent bullhealthy

Duration bullTemporary bullChronic

Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

CARRIERS

A Carrier is defined as an infected

person or animal that harbors a

specific infectious agent in the

absence of discernible clinical

disease and serves as a potential source of infection for others

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Carriers

bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state

FEATURES OF CARRIER

bull Three elements have to occur to form a carrier state

1 The presence in the body of the disease agent

2 The absence of recognizable symptoms and signs of disease

3 The shedding of disease agent in the discharge or excretions

4 As a source of infection to others

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull Cases only in some diseases -- human reservoir ndash measles pertussis

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

CARRIERS

TYPE

A) Incubatory- AIDS

B) Convalescent- typhoid HepA

C) Healthy- hep B

DURATION

A)Temporary- polio

B)Chronic- GBS-enterica carriers

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

bull ACTIVE CARRIERldquo THE INDIVIDUAL HAS THE PATHOGEN PRESENT AND IS SHOWING THE SS OF THE DISEASE

bull CONVALESCENT CARRIER INDIVIDUAL HAS RECOVERED FROM THE SS BUT STILL HAS PATHOGEN PRESENT

bull INCUBATORY INDIVIDUAL HAS THE PATHOGEN PRESENT BUT THE INDIVIDUAL WILL NEVER SHOW THE SS

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

Asymptomatic Carrier

bull Infected but show no symptoms of disease example gonorrhea

bull

And Enjoy

And Enjoy